Recently, a friend's Twin Comanche had this stain. It was under the left wing, outboard of the main tank, and spilling across the gear door. Hoping against hope sort of like buying lottery tickets I removed the small plate where the auxiliary tank's nipple connects to the line going to the fuel selector. Dry as the desert sand.

Telling the plane's owner the bad news, I extracted the bladder. Removing this flexible fuel tank is a job that requires patience, an awareness of how it's connected to the wing, plus strong hands and fingers. Putting it back requires the same except more so. There are more methods of installation that I can cover here, and more than most of you could stand reading about, so we'll deal just with the Piper line, specifically the PA-30. But first let's see what we're dealing with and why. [back to top]

Rubber Tanks

The development of rubber fuel bladders for aircraft came about during World War II. With the development of bigger and heavier aircraft that flew faster and higher and vibrated a lot more it became apparent that something flexible was needed to hold the increasing quantities of fuel. There was also a need for ease of maintenance (i.e., removal and replacement). So what better than rubber?

Rubber--Crude and Otherwise

Rubber bladders, or tanks, aren't rubber at least real rubber. Pure crude rubber is a white or colorless hydrocarbon, the simplest unit of which is isoprene (chemical formula C5H8).

At very low temperatures, about 195 degrees C (-319F), crude rubber is a hard, transparent solid; from 0 degrees to 10 degrees C (32 degrees to 50 degrees F) it's brittle and opaque and above 20 degrees C (68 degrees F) it becomes soft, resilient and translucent. When it's mechanically kneaded or heated above 50 degrees C (122 degrees F), it becomes plastic and sticky; above 200 degrees C (392 degrees F) it decomposes. Other attributes include it being insoluble in water, alkali and weak acid, but soluble in benzene, gasoline, chlorinated hydrocarbons and carbon disulfide.

I'm sure we all know that rubber comes from trees, but or those of you who insist on details: In its natural state rubber exists as a colloidal suspension in the latex of rubber-producing plants. The most important of these plants are the tree Hevea Brasiliensis of the spurge family and other species in the same genus, which were the sources of the original South American rubber, the commercially important Para rubber. The term Para rubber was then also applied to the product cultivated in rubber plantations of Indonesia, the Malay Peninsula and Sri Lanka. These trees produce about 90 per cent of all the new natural rubber consumed.

In the United States, rubberized goods were popular by the 1830s, most notably in the form of bottles and shoes. However, these products became brittle in cold weather, and tacky and malodorous in summer. In 1834 German chemist Friedrich Ludersdorf and American chemist Nathaniel Hayward discovered that the addition of sulfur to gum rubber lessened or eliminated the stickiness of finished rubber goods. In 1839 American inventor Charles Goodyear, using the findings of two chemists, discovered that cooking rubber with sulfur removed the gum's unfavorable properties in a process called vulcanization. Vulcanized rubber has increased strength, elasticity and greater resistance to changes in temperature. It's impermeable to gases and resistant to abrasion, chemical action, heat and electricity.

The political and economic significance of natural rubber became evident when, during World War II, the supply from the Far East was terminated. This shortage accelerated the development of synthetic rubber, which can be defined as any artificially produced substance that resembles natural rubber in essential chemical and physical properties.

Such substances are produced by chemical reactions, known as condensation or polymerization, of certain unsaturated hydrocarbons. The basic units of synthetic rubber on monomers, which are compounds of relative low molecular weight that form the building units of huge molecules called polymers. After fabrication, the synthetic rubber is cured by vulcanization. The synthetic rubber, Nitrile, is the one that's universally used in aircraft fuel tanks.  [back to top]

Removal and Installation

In the PA-30 the auxiliary tank is accessed by removal of the rear half of the engine nacelle, which isn't really a nacelle, but a fairing that continues over the rear of the wing. This give access to a panel in the wing's skin that's then unscrewed and removed. Once this panel is off, the metal plate covering the hole through which the bladder is removed and is unbolted and set aside. The fuel-sending unit is in this plate, so care must be taken with the wires and unit itself. As an aside, we discovered a significant amount of rust on the plate and the sending unit, and we replaced both. The cause of the rust was a split in the gasket that allowed water to seep in. But that's a story for another day. It may behoove everyone to cast a glance at these hidden recesses periodically.

Once this plate is removed, and the gas filler port is likewise undone, it's a simple process to reach between the upper wing surface and the bladder to locate the small bayonet clips that hold the top of the bladder. These clips keep the bladder from falling down as the fuel is removed and causing creases, which are the bane of tank life. Most if not all, aircraft have some method of doing the same thing, whether it be bayonet clips or rods, like those used in some Cessna's. They should all be in place and secure. Anyway, it's just a matter of pulling them down and the tank is loose. Did I mention disconnecting the tank's nipple? This is best done with a sharp knife to cut the hose away from the nipple and then, obviously, replacing the hose. Jerking and/or twisting the hose will do more damage. You wouldn't want to reuse the hose anyway, especially at about $1 a foot.

So now that the bladder is out, what do you do? It's up to you and your circumstances. If you have talent, the requisite materials, an experimental airplane and access to an autoclave, you can probably patch the hole yourself. If, however, like me, you have none of the above, you find a repair station. Your local FBO will know where there';s one in your area. We went to Aero-Tech Services of Santa Fe Springs in Southern California.

Before making the trek, we opted to make an inspection in hope that it wouldn't be necessary. I, along with a number of the local airport's experts, found what appeared to be one small hole with a stain radiating outward about 4 inches. No other problem areas could be seen.

For continuity, let's discuss the reinstallation. Basically it's a process of worrying the tank in place. But before you start, make sure that anything that may abrade the tank is covered with tape. This includes rivet heads, seams actually anything that isn't an undisturbed flat piece of metal. There's a specification in FAR25.853 that spells out the type of tape required, though it certainly looks a lot like duct tape. For those of you who've done it, your feeling that the new tank was bigger was correct. They're built slightly oversize because they shrink with time. The resultant folds are reportedly not a problem. Lay the tank in the cavity as smoothly as possible, make sure all the clips or other types of hangers are in place and connect the fuel lines. Don't over tighten the clamps or force the nipple into a position that imparts stress on them. Nipple failure is one of the major failure modes other than age.  [back to top]

Aero-Tech Services

Aero-Tech Services was started 35 years ago by Henry Krug, who previously had worked for Goodyear, the largest manufacturer of fuel cells in the world. Krug saw the need for a smaller manufacturer that could do repairs, specialty jobs and make replacement tanks for the burgeoning GA market.

The shop is now run by Krug's stepson, Dennis King, who took the time to show us around and explain the operation. Over the years, the operation has expanded. It now encompasses about 20,000 square feet and has 30 employees. The product line has expanded also and includes aircraft (general aviation is still the main staple), racecars, vintage racers, boats and anything else that has to carry fuel. One interesting application was a small doughnut-shaped cell used by the Scandinavian countries in a remote-controlled boat that has a camera mounted on it. The boat is launched from a mother ship and plies the fjords, which are too shallow for the larger ship, looking for illegal immigrants from Russia and Eastern Europe.

Along with every imaginable plane in the GA fleet, they have molds for most of the war birds, experimentals and long-range ferrying tanks. There are really only two types of cells (urethane or synthetic) and lots of trade names. The rumor that some tanks aren't repairable is untrue; it's more a function of age and the damage. Goodyear, as mentioned, makes most of the new tanks, and the older Goodyear tanks are urethane. These cells had their good and bad points. They weren't as subject to ozone damage, but the nipples could be damaged by ham-fisted mechanics. The nipples are now reinforced, and this problem has gone away. While many of you heard of a tank manufacturer named Loral, it's now called Engineered Fabrics, but it was and is part of Goodyear only the name changed.  [back to top]

The Repair

Full of hope and confidence that this would be a simple repair, we watched as the tank was cleaned, had a solvent poured inside, was sealed with plates and was pressurized with air. There must have been a dozen leaks on the top of the cell, and according the person doing the test, the entire top face should be scrapped. The bottom, we were told, rarely leaks, because there's always fuel covering it. So what to do? Get if repaired or opt for a new tank?

Depending on the area involved, there are two repair methods. For smaller areas, there's a vulcanizing process, where a patch is installed between two hot plates. These plates bring the material to 260 degrees to bond everything together. This was out of the question for our situation. The other process, used on large areas, is to reline the tank with a new sheet material. Repairs of this sort will last the life of the tank. This relining procedure isn't the same as a recoating process that has material applied like paint. Recoating is said to leave the possibility of detritus at some point in the future.

Not that is was applicable to our problem, but nipples can be replaced either by vulcanizing or a cold-bonding process. The more we talked, the more it seemed we'd have to go with a new tank, one of which was in stock.  [back to top]

The Manufacturing Process

It seems intuitive that not all rubber cells can be repaired, as was our case. So the manufacturing process was of great interest, actually every manufacturing process is of great interest. The first step in manufacturing a new bladder is making a duplicate tank out of cardboard. During the past 30 years, the company has made every tank imaginable, so it has the templates and drawings readily available. If that's not the case, it can use the tank that's brought in, dimensions supplied by the customer, scrap wings and fuselages or travel to the customer's aircraft and get the measurements on site.

Once the three-dimensional model is made, it's sprayed with a release agent. The first items applied are the fuel and vent nipples and any areas requiring special reinforcing. Each 'face' or side of the tank is then applied separately with two layers of a .022-inch-thick rubber called Nitrile. This is a fuel-barrier material that's cemented and rolled on. The material is also pliable enough to be shaped on the corners. All the critical areas are triple-reinforced, and the faces are overlapped.

Once wrapped in the Nitrile, it's overlaid with a rubber-coated nylon fabric that gives it strength, shape and resistance to outside agents. Both interior and exterior materials are rolled extensively to remove air and ensure the bond between all the layers of material. The process is labor-intensive and critical. When everything meets the inspector's acceptance, the unit is place in an autoclave at 80 psi and 260 degrees F for three hours. The finished product is now ready for cleaning, the first step of which is the removal of the cardboard. Hot water and manual labor are the ingredients in this operation. Once most of the cardboard is removed, the bladder is turned inside out and given a scrupulous cleaning and inspection. The employees seem to know the need for exactness in this task because of the care they were taking; even when they didn't think I or anyone else was watching. The seams are rubbed repeatedly to remove the smallest piece of debris, and all lap joints are twisted and turned in an attempt to locate the slightest opening or loose fit. Once they';re satisfied with the product, the various fittings are applied; the unit is sealed and tested, and shipped to the customer with a five-year warranty. Repairs usually average about $300, while new tanks for the majority of the Piper line run between $700 and $900.  [back to top]

Preventative Maintenance and Life Expectancy

What can pilots do to extend the fuel cell's life? Not too much, as it turns out, other than keeping the tanks full. Some planes have shallow tanks in the area of the filler, and it's possible to hit the bottom with the fuel nozzle. This area is always reinforced, but the pilot should be aware of it and avoid any physical damage along these lines. Keeping the plane in a hangar to avoid the extreme temperature swings between night and day is also good practice, though the added cost alone is probably not worth it. Periodically checking that the clips are still in place (mandatory on the Comanche each annual) may keep a crease always a weak point from appearing. But basically, most problems, such as aging, are nothing special, not unlike the sidewalls on tires.

The life expectancy on a new cell is 10 to 15 years, partly because the new unleaded fuels have agents that are harder on the cells. And for those of you using auto gas, be aware that it has twice the amounts of these harsh agents. On the flip side, the use of kerosene-based diesel and jet fuels causes less deterioration. As for switching fuels, this doesn't seem to be a problem either.

These tanks are available directly from Aero-Tech Services, FBOs and distributors worldwide. Don't forget to mention that you're a member of the International Comanche Society or one of the other organizations to get a good discount.   [back to top]